In recent years, the development of display devices using light emitting devices including electroluminescent (EL) devices has been conducted actively. The light emitting device has high visibility because it emits light for itself. It does not need a back light that is needed in liquid crystal display devices (LCD), and thus it is suitable for forming items that have a low profile and have nearly no limits to the field of view.
Here, the EL device is a device having a light emitting layer that can obtain luminescence generated by applying an electric filed. The light emitting layer has light emission (fluorescence) in returning from the singlet excited state to the ground state, and light emission (phosphorescence) in returning from the triplet excited state to the ground state. In the invention, the light emitting device may have any light emission forms above.
The EL device is configured in which the light emitting layer is sandwiched between a pair of electrodes (an anode and a cathode), forming a laminated structure in general. Typically, a laminated structure of the anode/hole transport layer/emissive layer/electron transport layer/cathode is exemplary. Furthermore, there are the other structures laminated between an anode and a cathode in the order of the hole injection layer/hole transport layer/light emitting layer/electron transport layer, or hole injection layer/hole transport layer/light emitting layer/electron transport layer/electron injection layer. As the EL device structure used for the light emitting device in the invention, any structure described above may be adapted. Moreover, fluorescent pigment may be doped into the light emitting layer.
In the specification, the entire layers disposed between the anode and the cathode are collectively called the EL layer in the EL element. Accordingly, the hole injection layer, the hole transport layer, the light emitting layer, the electron transport layer, and the electron injection layer are all included in the EL element. The light emitting element formed of the anode, the EL layer, and the cathode is called EL element.